Irradiating Coil And Magnetic Resonance Imaging Apparatus Using The Same

ABSTRACT

An irradiating coil includes a first ring conductor located on a plane, a second ring conductor having a smaller diameter than a diameter of the first ring conductor and located substantially on the plane of the first ring conductor, a plurality of first line conductors connected the first ring conductor and to the second ring conductor, and an arrangement for making uniform a radio-frequency-pulse that is transmitted by the irradiating coil. The pulse uniforming arrangement includes a diverger which diverges a current flow of the first line conductors.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 10/451,910, filed Jun. 27, 2003, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a magnetic resonance imaging apparatusof the type that creates an image of desired portions of an object to beexamined; and, more particularly, the invention relates to anirradiating coil in such an apparatus for applying a radio-frequencypulse to the object.

BACKGROUND OF THE INVENTION

A magnetic resonance imaging apparatus (hereinafter referred to as anMRI apparatus) is designed to measure the concentration distribution andthe relaxation time distribution of nuclear spins at a desired portionto be examined within an object to be examined, utilizing the nuclearmagnetic resonance phenomenon, and to display an image of a crosssection of the object obtained from the obtained data.

The nuclear spins of an object, which is placed within an apparatus forgenerating a uniform and strong magnetic field, perform precession at afrequency (Lamor frequency) determined by the magnetic field strengtharound an axis in the direction of the magnetic field. When aradio-frequency pulse having a frequency equivalent to the Larmorfrequency is irradiated to the object from the outside, the nuclearspins are excited and shift to a higher energy state (nuclear magneticresonance phenomenon). When the radio-frequency pulse irradiation to theobject is terminated, the nuclear spins return to a low energy state ata time constant in accordance with their respective states, and theyradiate electromagnetic waves (NMR signal) to the outside of the object.The electromagnetic waves are detected by a radio-frequency receivingcoil, which is synchronized with their frequency.

Here, gradient magnetic fields on x-, y-, and z-axes are applied to themagnetic field space so as to add positional information to the NMRsignals. Consequently, the positional information within the space canbe obtained as frequency information.

For radio-frequency pulse irradiation, an irradiating coil is used whichgenerates a radio-frequency magnetic field in a direction perpendicularto the static magnetic field direction. This irradiating coil has beenresearched and modified to obtain an improvement of the irradiationuniformity over a wide area in the magnetic field space, and varioustypes of coils have been used.

FIG. 1 is a diagram showing one example of an irradiating coil, whereinan example of a planar birdcage coil is shown. Referring to FIG. 1, twoconcentric ring conductors 1 a and 1 b having different sizes on thesame plane are mutually connected by a plurality of line conductors 2,extending in the radial direction.

FIG. 2 is an equivalent circuit diagram of the planar birdcage coil.Referring to FIG. 2, a loop a and a loop b designate the ring conductor1 a and the ring conductor 2 a, respectively. The loop a and the loop bare usually synchronized at a magnetic resonance frequency. For thesynchronization, condensers C and coils L are used.

FIG. 3 is a diagram showing the voltage distribution and currentdistribution in the loop a shown in FIG. 2. Referring to FIG. 3, thecurrent is maximized and the voltage is minimized at the feeding pointsd, since the loop a is synchronized at the resonance frequency.

FIG. 4 is a diagram showing one example of the irradiating planarbirdcage coil shown in FIG. 1, which is mounted in an MRI apparatus.Referring to FIG. 4, an irradiating coil 18 is usually located in thevicinity of an object 14 to be examined so as to efficiently apply anirradiation pulse to the object 14. Around the object 14, there arearranged a receiving coil 17 for receiving magnetic resonance signalsthat are generated from the object 14, a pre-amplifier 22 for amplifyingthe received signals, distribution lines 23 for connecting the signalsthat are amplified by the pre-amplifier 22 to an A/D converter (notshown), and the like, which are usually mounted in a bed on which theobject 14 is supported. The receiving coil 17, the pre-amplifier 22, andthe distribution lines 23 may be arranged in one location, for ease ofmanufacturing and operationality.

FIG. 5(B) is a graph showing an intensity distribution of radiation thatis applied to the object, the intensity having been measured atarbitrary points (c, d, e, f, g, h, i, and j, along the phantom linecircle in FIG. 5(A)) on the line conductors 2 of the above-mentionedplanar birdcage coil. In FIG. 5(B), the vertical axis represents theirradiation intensity, and the horizontal axis represents locationsalong the phantom line circle. As shown in FIG. 5(B), the irradiationintensity is larger on the line conductors 2 than in the space betweenthe line conductors 2, and it has a pulse shape that is pulsing with auniform width.

As shown in FIG. 4, in the MRI apparatus, imaging is performed while theobject 14, on which the receiving coil 17 is placed, is mounted on thebed. The pre-amplifier 22, for amplifying the signals sent from thereceiving coil 17, and the distribution lines 23 are installed in thebed.

To adjust the location of the object 14 so as to image a particularportion, the bed is constructed so that it can be moved. However, thepositional relation between the pre-amplifier 22, the distribution lines23 and the irradiating coil 18 is changed by moving the bed, since thepre-amplifier 22 and the distribution lines 23 are installed in the bed.Consequently, a potential difference is generated between the potentialof the receiving coil 17, the pre-amplifier 22, and the distributionlines 23, and that of the irradiating coil 18, and so a radio-frequencycoupling is generated. The strength of the radio-frequency couplingfluctuates when the positional relation is changed, since the positionalrelation affects the potential difference.

Next, the fluctuation of the radio-frequency coupling will be describedwith reference to FIG. 6. Referring to FIG. 6, the irradiating coil 18operates as a QD coil. Here, reference number 8 represents a condenser Cin the circuit diagram of FIG. 2, and it is located where the voltage ismaximized in FIG. 3.

Usually, in a QD coil, one feeding point does not affect a resonantcircuit of another feeding point, since the two feeding points areperpendicularly arranged. However, as shown in FIG. 4, when only oneside of the irradiating coil is subject to radio-frequency coupling dueto the effect of the distribution lines 23 (in FIG. 6, the pre-amplifier22 and the distribution lines 23 are arranged as shown) or the like, thepre-amplifier 22 and the distribution lines 23 are located where thepotential of the irradiating coil 18 is high. In this case,radio-frequency coupling between the pre-amplifier 22, the distributionlines 23, and the irradiating coil 18 is caused to fluctuate withfluctuation of the positional relation between the pre-amplifier 22 andthe distribution lines 23.

Next, this fluctuation of the potential will be described with referenceto FIG. 7. FIG. 7 shows a voltage that is generated in resonant circuitsd and d′ in FIG. 6. The respective voltage distributions of circuits dand d′ are indicated by the v1 and v1′ solid lines in ideal conditions.The potential of v1′ at a point where the potential of v1 is highest (orlowest) is the base potential, and this prevents radio-frequencycoupling. However, when the object 14 is moved, or the bed is moved, thepre-amplifier 22 and the distribution lines 23 are moved relative to theirradiating coil 18. They are moved to a location where the potential ofthe resonant circuit d is the highest, whereby the potential is shiftedas shown in FIG. 7 (represented by the broken line). Consequently, thelocation where the potential of d is a base potential is shifted fromthe point e, whereby fluctuation of a potential difference between d andd′ is generated. Consequently, the orthogonality of the QD coils d andd′ between d and d′ is lost, and a radio-frequency coupling is generatedbetween them.

An error in the phase of the radio-frequency magnetic field generatedfrom the two coils of the QD coil occurs, varying from 90 degrees due tothe above-described radio-frequency coupling, and, consequently, theuniformity of the radiation pulse and the irradiation efficiency aredeteriorated. However, in the conventional technique, consideration forthe radio-frequency coupling between the irradiating coil 18 and thedistribution lines 23 in the vicinity thereof has not been given.

Also, in the planar birdcage coil, line conductors 2 are radiallyarranged between the two ring conductors 1 a and 1 b, which havedifferent sizes. Consequently, the irradiation intensity is large in thevicinity of the central portion close to the ring conductor 1 b, sincethe distance between two line conductors is narrow. On the contrary, theirradiation intensity is small on the periphery close to the ringconductor 1 a, since the distance between two line conductors is broad.

That is, the irradiation intensity distribution is lower where the lineconductors 2 are farther from the ring conductor 1 b, whereby theoverall irradiation distribution is not uniform. Therefore, when imaginga relatively large object, the sensitivity in the portions other thanthe central portion in the obtained image is lower, in comparison withthe central portion.

Thus, in the conventional technique, consideration to provide a uniformirradiation pulse has not been given.

SUMMARY OF THE INVENTION

The present invention is made in consideration of the above-describedcircumstances, and the main object thereof is to further improve theimage quality of an MRI image.

Another object of the invention is to provide an irradiating coil thatcan lower the proportion of radio-frequency coupling between anirradiating coil and distribution lines or the like, improve theirradiation uniformity and the irradiation efficiency, and produce agood-quality image, and an MRI apparatus using such an irradiating coil.

A further object of the invention is to provide a birdcage coil that canimprove the sensitivity of the obtained image by irradiating aradio-frequency pulse which is uniform over the area to be imaged, andan MRI apparatus using such a birdcage coil.

Specifically, the irradiating coil in a typical example of the presentinvention includes a first ring conductor, a second ring conductorhaving a smaller diameter than that of the first ring conductor, and aplurality of first line conductors connecting the first ring conductorand the second ring conductor, wherein means for causing a uniformirradiation pulse to be transmitted by the irradiating coil is providedon the first ring conductor or on the first line conductors.

This means for causing a uniform irradiation pulse to be transmitted mayhave a plurality of resonant capacitance elements on the first ringconductor between each of the first line conductors. Alternately, it mayhave a resonant capacitance element on the first ring conductor betweeneach of the first line conductors, and also have a plurality of resonantcapacitance elements in a space between two specific line conductors.Further, means may be provided for lowering the potential at a positionshifted by 90 degrees from a feeding position, to which theradio-frequency signal for transmitting an irradiation pulse issupplied.

Further, the means for causing a uniform irradiation pulse to betransmitted may include a diverging means for diverging current that istransmitted to the first line conductors, which may have second lineconductors that are connected perpendicular to the first line conductorsand third line conductors for connecting both ends of the second lineconductors and the second ring conductor. This means may also beconstructed such that the second line conductors are connected to theapproximate center of the first line conductors at right angles. It mayalso be constructed such that the length of the third line conductorsand the distance from the second ring conductor to the connecting pointof the second line conductors to the first line conductors aresubstantially equal. Further, it may be constructed such that the secondline conductors are connected to the first line conductors two-thirds ofthe length of the first line conductors from the second ring conductorat right angles.

Further, the diverging means may include at least one resonantcapacitance element connected between each of the third line conductors.It may also include fourth line conductors that are connected betweenthe first line conductors and the second ring conductor perpendicular tothe first line conductors, both ends of which are connected to the thirdline conductors. It may also include a resonant capacitance element onthe first line conductors between each of the second line conductors andthe first ring conductor. It may also include fifth line conductors thatare connected between the second line conductors and the third lineconductors at a predetermined angle.

Further, the first ring conductor may consist of straight-line segmentsbetween each of the first line conductors, and the second ring conductoralso may consist of straight-line segments between the each of the firstline conductors. Means for diverging the operating voltage may also beincluded. Further, the irradiating coil may include a first ringconductor, a second ring conductor having a smaller diameter than thatof the first ring conductor, a plurality of first line conductors thatare connected to the circumferences of the first and the second ringconductors so as to equally divide the circumference of the first andthe second ring conductors, second line conductors extending in thecircumferential direction of the first and the second ring conductors,and at least one third line conductor connecting each end of the secondline conductors and the second ring conductor.

Further, an MRI apparatus, in one typical example of the presentinvention, includes a magnetic circuit for applying a static magneticfield to an object to be examined; gradient magnetic field coils forapplying to the object a slicing magnetic field, a readout gradientmagnetic field, and an encoding gradient magnetic field; an irradiatingcoil for applying an irradiation pulse that generates magnetic resonancein the object; a receiving coil for detecting a magnetic resonancesignal; and an image reconstructing means for obtaining an image of theobject using the signal detected by the receiving coil, wherein theirradiating coil has a first ring conductor, a second ring conductorhaving smaller diameter than that of the first ring conductor, aplurality of first line conductors for connecting the first ringconductor and the second ring conductor, and means for causing theirradiation pulse to be uniform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the structure of a conventional birdcage coil;

FIG. 2 is a schematic circuit diagram of a conventional planar birdcagecoil;

FIG. 3 is a diagram of the operating potential and current of a birdcagecoil;

FIG. 4 is a schematic cross-sectional view of a birdcage coil and itsperiphery;

FIG. 5(A) is a diagram of a birdcage coil, and FIG. 5(B) is a graph ofthe irradiation sensitivity distribution of the planer birdcage coil ofFIG. 5(A) according to the conventional technique;

FIG. 6 is a schematic diagram of a birdcage coil and a pre-amplifiercausing interference on the periphery thereof;

FIG. 7 is a diagram of the potential shift in the case where therelative position of a pre-amplifier or the like and a birdcage coil isshifted;

FIG. 8 is a schematic block diagram of an MRI apparatus according to thepresent invention;

FIG. 9 is a diagram of the structure of a planar birdcage coil accordingto a first embodiment of the present invention;

FIG. 10 is a potential graph of a planar birdcage coil according to afirst embodiment of the present invention;

FIG. 11 is a schematic diagram of the structure of a birdcage coilaccording to a second embodiment of the present invention;

FIG. 12(A) is a diagram of a birdcage coil, and FIG. 12(B) is a graph ofthe irradiation sensitivity distribution of the planar birdcage coil ofFIG. 12(A) according to the second embodiment of the present invention;

FIG. 13(A) is a schematic diagram of an improved birdcage coil accordingto the second embodiment of the present invention, and FIG. 13(B) is agraph of the irradiation sensitivity distribution of the birdcage coilof FIG. 13(A);

FIG. 14 is a schematic diagram of a planar birdcage coil according to athird embodiment of the present invention, wherein resonant capacitanceelements are distributed;

FIG. 15 is a graph of the irradiation sensitivity distribution of aplanar birdcage coil according to the third embodiment of the presentinvention;

FIG. 16 is a schematic diagram of a planar birdcage coil according afourth embodiment of the present invention; and

FIG. 17 is a schematic diagram of variations of planar birdcage coilaccording to other embodiments of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, various embodiments of the present invention will bedescribed in detail with reference to the drawings.

FIG. 8 is a block diagram of an MRI apparatus according to the presentinvention. Referring to FIG. 8, the MRI apparatus, which is designed toobtain a tomogram of an object 14 to be examined utilizing nuclearmagnetic resonance (NMR), includes a magnetic field generating device11, an MRI unit 12, gradient magnetic field coils 21, irradiating coils18, receiving coils 17, a bed 16, and a display device 15.

The gradient magnetic field generating device 11 is designed to generatea strong and uniform static magnetic field towards the object. Magneticfield generating means of a permanent magnet type or of asuperconductive type is installed in the space around the object 14. Amagnetic field generating device 11 of the vertical magnetic field typeis preferably employed, but magnetic field generating devices 11 of thehorizontal magnetic field type can also be used.

The MRI unit 12 includes a control unit 10 for controlling various pulsesequences in imaging, a high-speed image data calculating device 13, agradient magnetic field power source 20, and a radio-frequency device19.

Three pairs of the gradient magnetic field coils 21 are locatedrespectively on an x-axis, a y-axis, and a z-axis, and they form thenecessary gradient magnetic fields around the object 14 with outputcurrent of the gradient magnetic field power source 20 that iscontrolled by the control unit 10, thus adding positional information toNMR signals. The radio-frequency device 19 irradiates to the object 14 aradio-frequency pulse for exciting nuclear spins in accordance withcontrol by the control device 10. NMR signals generated by nuclear spinexcitation are detected by the receiving coils 17, and they arecollected by the radio-frequency device 19. These signals are subject toimage-reconstruction calculation and the like at the calculating device13 and are made into an MRI image, which is displayed by the displaydevice 15.

Next, the irradiating coils 18, according to a first embodiment of theinvention, will be described with reference to FIG. 9. Each of theirradiating coils 18 includes a ring conductor 3, a ring conductor 5having smaller diameter than that of the ring conductor 3, and lineconductors 4 connected at even intervals along the circumferences ofeach of the ring conductor 3 and the ring conductor 5. The ringconductor 3 and the ring conductor 5 are thus mutually connected.

The number of line conductors 4 is eight in this embodiment, but itneeds to be 4n (n is a natural number) when a quadrature transmission(QD transmission) system is used. Usually, the number of line conductors4 is selected from among four, eight, and sixteen. However, theinvention can also be applied to an irradiation coil including sixteenor more line conductors 4.

Resonant capacitance elements 8 are serially arranged on thecircumference of the ring conductor 3 between the respective lineconductors 4. Similarly, resonant capacitance elements (not shown) arealso arranged serially on the circumference of the ring conductor 5between the respective line conductors, and thus the irradiating coil isconstructed.

Here, radio-frequency signals of the radio-frequency device 19 areprovided to a feeding point 30 and a feeding point 31 of the irradiatingcoil 18. A plurality of resonant capacitance elements are seriallyarranged at the positions where the voltage is highest when viewed fromthe feeding points 30 and 31 (a position turned 90 degrees, which is aposition where resonant capacitance elements 32 and 33 are arranged). InFIG. 9, the resonant capacitance 32 (33) has three resonant capacitanceelements, but any plural number of resonant capacitance elements thatform the resonant capacitance 32 (33) is acceptable.

FIG. 10 is a graph showing the potential at positions of the irradiatingcoils shown in FIG. 9. In FIG. 10, the waveform 41 of the potential(broken line), in the case where the resonant capacitance 32 is formedof a plurality of condensers, fluctuates more gently than the waveform40 of potential (solid line), in the case where the resonant capacitance32 is formed of one condenser, whereby the resonant capacitance 32 thatis formed of plural condensers can reduce the fluctuation ofradio-frequency coupling among the pre-amplifier 22, the distributionlines 23 and the like in the arrangement of FIG. 4. That is, even if therelative positions of the pre-amplifier 22, the distribution lines 23and the irradiating coil 18 are changed, causing the potential 41 tofluctuate, since the waveform of the potential 41 is gentle, thepotential fluctuation thereof at the point e, for example, is smallerthan the potential 40.

Next, a second embodiment of the present invention will be described indetail with reference to FIG. 11 to FIG. 13.

Referring to FIG. 11, an irradiating coil 18-1 includes a ring conductor3, a ring conductor 5 having smaller diameter than the ring conductor 3,and a plurality of line conductors 4 connected at points along thecircumferences of the ring conductor 3 and of the ring conductor 5, inthe same manner as in the first embodiment. The number of the lineconductors 4 is eight, similar to the first embodiment. This numberneeds to be 4n (n is a natural number) when a quadrature transmission(QD transmission) system is used. The number of the line conductors 4preferably is four, eight, or sixteen.

Resonant capacitance elements 8 are serially connected along thecircumference of the ring conductor 3 between the respective lineconductors 4. Similarly, resonant capacitances 9 are also connectedserially along the circumference of the ring conductor 5 between therespective line conductors 4. The irradiating coil 18-1 is thusconstructed. In addition, line conductors 6 are connected to arbitrarypositions on the line conductors 4 so as to intersect with the lineconductor 4 at right angles. Each end of the line conductors 6 isconnected to the ring conductor 5 by a line conductor 7. In FIG. 11, thenumber of line conductors 7 is two, but the invention is not limitedthereto. As the number of the line conductors 7 is increased, theuniformity of the irradiation intensity is further improved. However,one to three is appropriate.

FIGS. 12(A) and 12(B) show the intensity distribution of radiationapplied to the object measured at arbitrary positions on the lineconductors 4 of the irradiating coil 18-1 (along the dashed linecircle). FIG. 12(B) shows the irradiation intensity distribution alongthe phantom line in FIG. 12(A). The solid line in FIG. 12(B) shows theirradiation intensity distribution in the case where the irradiationcoil of the second embodiment is employed, and the broken line in FIG.12(B) shows the irradiation intensity distribution in the case where theirradiation coil of the conventional technique (one without the lineconductors 6 and 7) is employed.

By connecting a plurality of the line conductors 7 to the lineconductors 4 via the line conductors 6, the current transmitted to theline conductors 4 is diverged to the line conductors 7 and thedistribution of the generated radio-frequency magnetic field can be madefurther uniform, whereby the irradiation intensity can be made moreuniform.

That is, when comparing the case where the current transmitted to theline conductors 4 is diverged to the line conductors 7 (actual line inFIG. 12(B)) with the case where the current transmitted to the lineconductors 4 is not diverged (broken line in FIG. 12(B)), the value ofthe current transmitted to each conductor at positions along the dashedline in FIG. 12(A) is smaller and the irradiation intensity is alsosmaller, when the current is diverged in comparison with the case wherethe current is not diverged. However, since the intervals between eachconductor are lessened, a drop in the irradiation intensity at thespaces between conductors is also reduced.

Consequently, the difference between the maximum and the minimum of theirradiation intensity is smaller when the current is diverged than whenit is not diverged, and, therefore, the uniformity of the irradiationintensity can be improved.

Next, an irradiating coil 18-2, that is made by extending the length ofthe line conductors 7 of the irradiating coil 18-1, will be described asa variation of the second embodiment, with reference to FIG. 13.

In the irradiating coil 18-1 in FIG. 12, the line conductors 6 areconnected to the approximate center of the line conductors 4, and thelength of the line conductors 7 are about half of that of the lineconductors 4. On the other hand, in the irradiating coil 18-2 in FIG.13, the line conductor 6-1 are connected at a point about two-thirdsalong the length of the line conductors 4 from the ring conductor 5 withthe smaller diameter, and the length of the line conductors 7-1 is thusabout two-third of that of the line conductors 4.

FIG. 13(B) shows the irradiation intensity distribution of the lineconductors 4 of the irradiation coil 18-2 measured where the lineconductors 4 pass a circle of arbitrary radius, wherein the radiationintensity distribution of the irradiating coil 18-2 is represented bythe actual line, and that of the irradiating coil 18-1, shown in FIG.12, is represented by a broken line.

As is clear from FIG. 13(B), the range of fluctuation of the irradiationintensity of the irradiating coil 18-2 at this circle, which is furtherremoved from the center, is smaller than that of the irradiating coil18-1. That is, when using the irradiating coil 18-2 shown in FIG. 13,the uniform irradiation intensity area can be further expanded incomparison with that when using the irradiating coil 18-1

As described above, the length of the line conductors 7 and 7-1 can beset when manufacturing the coil according to diagnostic functions andthe performance desired for the MRI apparatus.

According to the second embodiment, the current transmitted to the lineconductors 4 is more evenly distributed on the plane of the coil, sincethe current is diverged to the line conductors 7 (7-1). Consequently,the uniformity of the irradiation intensity is improved. Therefore, abirdcage coil that can uniformly transmit a radio-frequency pulse in thearea to be imaged can be realized, and an MRI apparatus with improvedsensitivity in the obtained image can be realized.

Next, a third embodiment of the present invention will be described withreference to FIG. 14 and FIG. 15. In the third embodiment, resonantcapacitance elements 100 are connected between line conductors 7 (7-1)of the irradiating coil 18-1 (18-2) in the second embodiment, and theirradiating coil 18-3 is thus constructed.

FIG. 15 shows the irradiation intensity distribution on a line thatpasses through the resonant capacitance elements 100 that aredistributed between the line conductors 7 of the irradiating coil 18-3.In FIG. 15, the intensity distribution of the irradiating coil 18-3 ofthe third embodiment is represented by the solid line, and the intensitydistribution of the irradiating coil 18-1 of the second embodiment isrepresented by the broken line.

As shown in FIG. 15, a radio-frequency magnetic field is generated dueto current flowing through the resonant capacitance elements 100 bydistributing the resonant capacitance elements 100, whereby theirradiation intensity distribution on the line passing through theresonant capacitance elements 100 is more uniform.

As described above, according to the third embodiment of the presentinvention, the same effect as obtained by the second embodiment can beobtained. Further, a radio-frequency magnetic field is generated due tocurrent transmitted to the resonant capacitance elements 100 bydistributing the resonant capacitance elements 100, whereby theirradiation intensity distribution on the lines that pass through theresonant capacitance elements 100 can be made more uniform.Consequently, the overall irradiation intensity distribution can be moreuniform.

Incidentally, in the third embodiment, two resonant capacitance elements100 are connected between two adjacent line conductors 7. However, thenumber of the resonant capacitance elements 100 to be connected is notlimited to two. As the number of the resonant capacitance elements 100is larger, the irradiation intensity distribution can be furtherimproved. The number of the resonant capacitance elements 100 can besuitably set depending on the length of the line conductors 7, the sizeof the resonant capacitance elements 100, and the like.

Next, an irradiating coil 18-4, according to a fourth embodiment, willbe described with reference to FIG. 16. In the fourth embodiment, thecoil is constructed such that line conductors 6′, which areperpendicular to the line conductors of the irradiating coil 18-1according to the second embodiment and both ends of which are connectedto the line conductors 7, are arranged between the line conductors 6 andthe ring conductor 5.

By providing such a structure, the line conductors 6′ can divergecurrent transmitted to the line conductors 4, whereby the uniformity ofthe irradiation intensity is further improved compared to that obtainedin the second embodiment. Incidentally, in FIG. 16, one line conductor6′ is added to one line conductor 4. However, the number of the lineconductor 6′ is not limited to one, and two or more can be added. Thecurrent can be further diverged by increasing the number of the lineconductors 6′, whereby the uniformity of the irradiation intensity canbe further improved.

Next, other embodiments of the irradiating coil according to theinvention will be described with reference to FIGS. 17(A) to 17(C).

First, an irradiating coil 18-5, as shown in FIG. 17(A), is constructedsuch that resonant capacitance elements 8′ are arranged on the lineconductors 4 instead of the resonant capacitance elements 8 of theirradiating coil 18-1 shown in FIG. 11, whereby the same effect asobtained by the resonant capacitance elements 8 and 9 can be obtained.Thus, the irradiation intensity can be made more uniform with a simplerstructure.

Next, the irradiating coil 18-6 shown in FIG. 17(B) is constructed suchthat the shape of the ring conductors 3 and 5 of the irradiating coil18-1 in FIG. 11 is that of a polygon, instead of a ring. That is, theshape of the ring conductors 3 and 5 constitutes a n-sided regularpolygon (n is a multiple of four) in accordance with the number of theline conductors 4. In this manner, when the form of the irradiating coilis a polygon, the birdcage coil is manufactured by combining lineconductors, and the conductors do not need to be made into a roundshape, whereby the manufacturing becomes easier. When the irradiatingcoil 18-6 in FIG. 17(B) is used, the irradiation intensity can be madeuniform in the same manner as the coil described in FIG. 11.

Incidentally, in accordance with the present invention, the term “ringconductors” includes conductors having a circular shape and conductorshaving a polygonal shape.

Next, an embodiment can be provided in which irradiating coil 18-7 inFIG. 17(C) is constructed such that the line conductors 6 of theirradiating coil 18-1 in FIG. 11 are shortened, and the line conductors6 a that connect the line conductors 6 and the line conductors 7 areprovided.

In order to equalize the current to be diverged from the line conductors4 to the line conductors 7 with the current remaining in the lineconductors 4, and to thus cause a uniform radio-frequency magnetic fieldto be generated, the line conductors 6 a for connecting the lineconductors 6 and 7 may be provided, such that the length between theconnecting point of the line conductors 4 and the line conductors 6 andthe ring conductor 5, and the sum of half the length of the lineconductor 6 and the length of the line conductors 7 are close to eachother. This is done because the difference between the value of currenttransmitted to the line conductors 4 and that transmitted to the lineconductors 7 becomes large when the difference between the lengthbetween the connecting point of the line conductors 4 to the conductors6 and the ring conductor 5 and the sum of half the length of the lineconductor 6 and the length of the line conductors 7 is large, since theconductors have a slight resistance.

The above-mentioned structure is designed to avoid such a phenomenon bymaking those two lengths close to each other, and to thus improve theuniformity of the irradiation intensity.

In this case, as one example of conditions for manipulating the lineconductors 6, 6 a, and 7 to be the same length as the line conductor 4,the angle made by the line conductor 6 a and the line conductor 6 is noless than 90 degrees and is less than 180 degrees, and the angle made bythe line conductor 6 a and the line conductor 7 is also no less than 90degrees and is less than 180 degrees. However, such a condition is notlimited to this example.

Incidentally, while each of the above embodiments has describedindividually, combinations thereof may also be employed. For example,when combining the embodiment shown in FIG. 16 and the one shown in FIG.17(B), the shape of the coil in FIG. 16 can be made into a polygon. Whencombining the embodiment shown in FIG. 9 and the one shown in FIG.17(B), the shape of the coil in FIG. 9 can be made into a polygon.

In the above-described embodiments, the resonant capacitance elements (8and 9) are provided to both the ring conductor 3 having a large diameterand to the ring conductor 5 having a small diameter. However, theresonant capacitance elements arranged on the ring conductor 5 having asmaller diameter may be omitted.

Further, the present invention is not limited to the above-describedembodiments, and various changes may be made within the scope of theinvention.

According to the present invention, a planar birdcage coil with a simplestructure that can transmit a irradiation pulse uniformly can berealized. More specifically, the planar birdcage coil is constructedsuch that a plurality of resonant capacitance elements are seriallyarranged at positions where the potential of the birdcage coil is high.In this manner, fluctuation of the operating potential can besuppressed, and the potential fluctuation in response to externalfactors can be reduced.

In this manner, a potential shift due to a change of arrangement of thepre-amplifier and the distribution lines with respect to the planarbirdcage coil can be reduced, and fluctuation of the radio-frequencycoupling can be suppressed. Consequently, an irradiating coil that canreduce the proportion of the radio-frequency coupling between theirradiating coil and the distribution lines or the like, improveirradiation uniformity and irradiation efficiency, and obtain an imagehaving good quality can be realized. Also, an MRI apparatus using thiscoil can be realized.

Further, since the coil is provided with line conductors and resonantcapacitance elements for diverging the current to be transmitted to theline conductors, an irradiating coil (a birdcage coil) that can transmita radio-frequency pulse uniformly in the area to be imaged can berealized. Also, an MRI apparatus in which the sensitivity of the imageto be obtained is improved can be realized.

1. An irradiating coil comprising: a first ring conductor located on aplane; a second ring conductor having a smaller diameter than a diameterof the first ring conductor and located substantially on the plane ofthe first ring conductor; a plurality of first line conductors connectedthe first ring conductor and to the second ring conductor; and means formaking uniform a radio-frequency-pulse that is transmitted by theirradiating coil; wherein the pulse uniforming means includes divergingmeans for diverging a current flow of the first line conductors.
 2. Anirradiating coil according to claim 1, wherein the diverging meansincludes at least one branch from the first line conductor to the secondring conductor.
 3. An irradiating coil according to claim 2, wherein thebranch is provided on both sides of each first line conductor.
 4. Anirradiating coil according to claim 2, wherein the branch is forbypassing a current flow between the first ring conductor and aconnecting point of the first line conductor with the branch.
 5. Anirradiating coil according to claim 2, wherein the branch has aplurality of connecting points with the first line conductor.
 6. Anirradiating coil according to claim 2, wherein the branch has a secondline conductor connected to the second ring conductor.
 7. An irradiatingcoil according to claim 6, wherein each current flowing on the secondline conductor and the first line conductor between the second ringconductor and a connecting point of the first line conductor with thebranch is substantially a same current.
 8. An irradiating coil accordingto claim 2, wherein two adjacent branches are disposed between twoadjacent first line conductors and are electrically connected to eachother via at least one resonant capacitor.
 9. An irradiating coilaccording to claim 8, wherein the electrical connections between thebranches and the resonant capacitors build up a circuit in acircumferential direction between the first ring conductor and thesecond ring conductor.
 10. An MRI apparatus comprising: a staticmagnetic field generating means for applying a static magnetic field toan object to be examined; a gradient magnetic field coil for applying tothe object a slicing gradient magnetic field, a readout gradientmagnetic field, and an encoding gradient magnetic field; an irradiatingcoil for applying to the object an irradiation pulse for generatingmagnetic resonance, including a first ring conductor located on a plane,a second ring conductor having a smaller diameter than a diameter of thefirst ring conductor and located substantially on the plane of the firstring conductor, and a plurality of first line conductors connecting thefirst ring conductor and the second ring conductor; a receiving coil fordetecting a magnetic resonance signal; and image reconstructing meansfor obtaining an image of the object using the signal detected by thereceiving coil; wherein the irradiating coil includes means for makingthe irradiation pulse uniform; and wherein the pulse uniforming meansincludes diverging means for diverging a current flow of the first lineconductors.
 11. An MRI apparatus according to claim 10, wherein thediverging means includes at least one branch from the first lineconductor to the second ring conductor.